qa: exclude `nofallback` mount option when using v1-style syntax

qa: exclude `nofallback` mount option when using v1-style syntax

Otherwise, certain upgrade tests fail which install pacific
or earlier releases since the mount helper does not understand
this mount option, thereby passing it to the kernel which would
does not handle this config causing mount to fail in tests.

Note that this mount config is only used during teuthology tests
to catch v2-style syntax implementation bugs in the kernel.

Fixes: http://tracker.ceph.com/issues/53487
Signed-off-by: Venky Shankar <vshankar@redhat.com>